[w3c/ServiceWorker] Reduce GC exposure (#1127)

According to @wanderview there's a couple places where service workers end up exposing GC, whereas elsehwere in the platform we've made every effort to avoid that.

I'd like to use this issue to ask @wanderview to explain it in more detail and then hopefully together find ways to preserve the GC invariants others have fought so hard for to keep.

cc @smaug---- @domenic 

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